In the advanced lithography process for the fabrication of semiconductor devices, a light source of shorter wavelength is used for exposure. A subsequent transition to lithography using extreme ultraviolet (EUV) is regarded promising. Since the EUV lithography uses a reflecting optical system, the lithography accuracy can be adversely affected even by a slight thermal expansion of each member (e.g., substrate) in the lithographic optical system induced by the heat that has reached there. Accordingly, members like reflecting mirrors, masks, and stages must be made of low expansion materials. Titania-doped quartz glass is known as a typical low expansion material. The addition of a certain amount of titania makes it possible to minimize the thermal expansion of quartz glass.
The EUV lithography members must also have a uniform distribution of low thermal expansion. To gain a uniform distribution of low thermal expansion, it is of the first priority that quartz glass be doped with titania in a uniform concentration. For example, JP-A 2004-315351 discloses titania-doped quartz glass in which a difference between maximum and minimum TiO2 concentrations is less than or equal to 0.06% by weight in a range of 30 mm×30 mm, and a variation (Δn) of refractive index which varies with the TiO2 concentration in quartz glass is less than or equal to 2×10−4 in a range of 30 mm×30 mm.
Also an OH group concentration in titania-doped quartz glass is known as one of the physical properties having impact on the low thermal expansion of titania-doped quartz glass. For example, WO 2005/114328 discloses a quartz glass blank having a mean OH content of 700 to 1,000 wt ppm, wherein a variation of OH content, averaged over a thickness of the quartz glass blank, does not exceed ±50 ppm in an area of the main functional direction. Then the optical and thermal properties of quartz glass are kept as homogeneous as possible.
JP-A 2005-022954 describes that the fictive temperature of glass is correlated to the extent of a zero expansion temperature range that is a temperature range in which the coefficient of thermal expansion (CTE) of glass becomes virtually zero (0). For the purpose of broadening the zero expansion temperature range, the fictive temperature is preferably up to 950° C., more preferably up to 900° C., and even more preferably up to 850° C. Since a high OH group concentration in glass indicates fast structural relaxation, the manufacture of a glass block having a large diameter enough to invite a temperature distribution tends to entail a fictive temperature distribution. Thus the OH group concentration is preferably up to 600 ppm, more preferably up to 400 ppm, and even more preferably up to 200 ppm. In addition, if the OH group concentration varies over a wide range, the structural relaxation time may substantially vary at different positions, inviting a difference in fictive temperature. Thus the variation of the OH group concentration in titania-doped quartz glass is preferably within 50 ppm, more preferably within 30 ppm, and even more preferably within 10 ppm.
As discussed above, the OH group concentration in titania-doped quartz glass has a significant impact on low thermal expansion. It is thus believed important to specify the absolute amount and distribution of OH group concentration in titania-doped quartz glass.
WO 2005/114328 also refers to birefringence. In the preferred embodiment, the maximum stress birefringence (SDB) at 633 nm perpendicular to the main functional direction (or cylinder axis) does not exceed 5 nm/cm, and a substantial portion of the maximum stress birefringence has a gradient that does not exceed 50 (nm/cm)/cm.
JP-A 2008-182220 describes a peak-to-valley striae level or RMS striae level of a glass material computed from a retardation proportional to the amount of stress in the material. It is necessary to reduce the peak-to-valley striae level or RMS striae level since the striae level has an adverse impact on the light transmission of lens or window elements made of glass.
WO 2006/004169 discloses a TiO2-containing silica glass wherein an internal transmittance per 1 mm thickness in a wavelength range of 400 to 700 nm is at least 70% and an internal transmittance per 1 mm thickness in a wavelength range of 300 to 3,000 nm is at least 70%. If the internal transmittance is less than 70%, there may be inconvenience in inspection or evaluation such that an inspection to control homogeneity or surface smoothness by a measuring equipment using a laser interferometer is less likely to be easily carried out. Further, in a case of a component to let visible light or infrared light pass therethrough, the transmitted light intensity decreases.
As discussed above, the low expansion materials, typically for use as EUV lithography optical members, must meet numerous physical property values in order that the material have low thermal expansion and surface smoothness.
When quartz glass is used as EUV lithography members, the glass must meet the uniformity of low thermal expansion. Factors that have an impact on the uniformity of low thermal expansion of titania-doped quartz glass include a titania dopant concentration, impurity concentration, fictive temperature and the like. It is possible to achieve the uniformity of low thermal expansion of the overall glass by offsetting these factors or by rendering each of these factors uniform.